Cattle nutrition is a complex subject with a lot of terms, acronyms and numbers that many people don’t fully understand. Luckily, most decisions around cattle nutrition in northern Australia can be made using best practice recommendations and guides developed by people who have the relevant expertise. However, whilst you may not have to do all the calculations yourself, it is useful to know what some of the terms on feed labels are referring to.
How to speak Nutrition — a guide to common terminology
The energy of a feed refers to the energy that becomes available to animals when the molecules are broken down during digestion. This energy is then used to power the normal functions of the animal, such as moving, growing, producing heat, etc. Different feeds contain different amounts of energy depending on their composition. The majority of energy in a feed comes from carbohydrates, with some also provided by protein and fat.
There are two types of carbohydrates; non-structural and structural.
Non-structural carbohydrates
These are simple sugars and starches which are highly and quickly digestible. The seed/grain typically has the highest non-structural carbohydrate content of the pant, followed by young leaves or “green pick”.
Structural carbohydrates
Structural carbohydrates include cellulose, hemi-cellulose and lignin that form the plant cell walls. As the name suggests, these give the plant ‘structure’. Cellulose and hemi-cellulose can be slowly digested by microorganisms in the rumen, but lignin cannot. Generally, as the plant matures and the proportion of stem (or ‘structural’ parts) increases, there is a higher content of the slow or non-digestible structural carbohydrates. Structural carbohydrates are more commonly termed ‘fibre’.
Metabolisable energy (ME)
Metabolisable energy (ME) describes the energy content of feeds, sometimes written as MJ ME/kg DM or similar. In this abbreviation, MJ refers to megajoules—the unit or currency used to measure energy.
1 MJ = 1,000,000 J
1 MJ = 1000 KJ
1MJ = 239 calories
‘Kg DM’ refers to kilograms of dry matter. This figure is important as different plants (or feeds) contain different amounts of water. Removing the water component allows the nutrient value of the plants and feeds to be compared equally.
Rather than using the total energy (gross energy) as a measure of how much energy a plant or feed will supply an animal, ME is used as not all the energy in a feed is digested entirely, some passes through the animal and out in the faeces. ME is the energy that is available for the animal to use after the un-used energy lost in faeces, urine and rumen function is taken out.
The available ME is then used up by the animal for heat regulation, maintenance functions, movement and production (e.g. growth).
Digestibility
Digestibility refers to the proportion of a plant that can be digested by cattle and have nutrients extracted from it. This is directly related to the amount of lignin (the un-digestible, structural carbohydrate) in plants which increases as the amount of ‘stem’ on the plant increases. This process is sometimes referred to as lignification, and is a major reason the nutritional value of pastures declines as they mature—the animals simply can’t digest a lot of it!
DMD% is one way you might see digestibility written. This stands for dry matter digestibility, expressed as a percentage, so that plants or feeds can be compared without confounding due to different moisture contents.
Protein
Protein is the ‘building block’ of the body, being a significant component of muscle (and milk. It also makes up enzymes that drive metabolism and other functions/reactions in the body.
Cattle can get protein from two sources: true protein and microbial protein.
True protein
True protein refers to protein that is consumed as protein, and then broken down either in the rumen by microbes (commonly known as rumen-degradable protein, or RDP), or chemically in the abomasum (rumen-un-degradable protein UDP, also called ‘bypass protein’). True protein comes from protein in the feed, which is highest in grain or young plants. Additionally, legumes are contain more protein than grasses. You may also see it referred to as feed or plant protein.
Microbial protein
Microbial protein refers to protein that is actually synthesised or created by microbes in the rumen. Microbes utilise nitrogen (in the form of ammonia) to grow themselves, creating microbial protein. When microbes die and are flushed from the rumen into the abomasum, they can be broken down chemically and used in the same way as true protein. Microbes consume the nitrogen from the feed, or supplement, that the animal has eaten and has been broken converted to ammonia in the rumen.
NPN
NPN is non-protein nitrogen. This is essentially nitrogen that can be fed to cattle and converted into microbial protein by microbes. Nitrogen is a major component of protein. In the dry season, non-protein nitrogen is commonly supplied to cattle in the form of urea supplements.
The total amount of protein that the feed can produce once digested by the rumen microbes can be calculated by multiplying the nitrogen content by 6.25. For example, urea is a NPN source containing no energy or true protein, but typically has a nitrogen content of 46%. This means, once microbes convert this NPN to microbial protein, 1 gram of urea can supply 2.87 grams of crude protein (0.46 x 6.25)!
Crude Protein (CP%)
You will come across ‘CP%’ in many nutritional tables. CP% stands for Crude Protein percentage, and is a measure of the total amount of protein in a feed. ‘Crude’ just means that it is inclusive of ‘protein’ that has not yet been converted to protein yet, i.e. nitrogen. This is because CP% is calculated using the nitrogen x 6.25 calculation mentioned above, and therefore measures nitrogen that is still in the form of, e.g. ammonia, as well as nitrogen that is in the form of true protein.
Table 1. Nutritional value of common feeds — an example of how dry matter % (DM %), digestibility, energy content (ME [MJ/kg DM]) and crude protein content (CP %) may be written.
| Feed | Description | DM (approx) (%) | Digestibility (%) | ME (MJ/kg DM) | CP (%) | |
|---|---|---|---|---|---|---|
| Grain | 90 | 90 | 13 | 8-12 | ||
| Molasss | 75 | 90 | 13 | 4.3 | ||
| Tropical grasses | Description | DM (approx) (%) | Digestibility (%) | ME (MJ/kg DM) | CP (%) | Tropical grass + legume |
| Phase 1 | Early, rapid growth | Low (<30% dry matter) | 70 | 10 | 10-16 | 12-16 |
| Phase 2 | Beginning to grow stem, mostly green | Medium (30-50%) | 60 | 8.5 | 8-10 | 10-12 |
| Phase 3 | Flowering and seed set, growth slows, 10-30% green | Medium / high (50-70%) | 55 | 7.5 | 6-8 | 7-10 |
| Phase 4 | Senescence, no growth, no green | High (>80%) | 50 | 6.5 | 3-6 | 7 |
More information about urea supplementation →
Minerals like phosphorus (P), calcium (Ca) and sulphur (S) are essential for various body functions. You may hear the terms ‘macro-minerals’ and ‘micro-minerals’ (also known as trace minerals). Macro-minerals are essential in larger quantities, micro-minerals are essential in smaller quantities. Usually these will be written as a percentage of the feed.
More information about phosphorus supplementation →
More information about minerals and vitamins →
The above terminology is mostly seen where the nutritional value or content of a feed is being described. If you are looking at the nutritional requirements of cattle, they will usually be reported as ‘per day’ rather than ‘per kg DM’. For example:
- Energy requirements may be written as: MJ ME/day
- Minerals may be written as: g/day
Protein will usually still be written as CP%. Generally, consuming the amount of feed needed to meet energy requirements, at the recommended CP%, will ensure protein requirements are met. Occasionally protein requirement will be written as grams of RDP/UDP per day.
To work out whether the nutritional content of a feed meets the nutritional requirements of an animal, you have to know how much that animal will physically eat per day and also the digestibility of that feed.
More information about nutrient requirements →
Written by Stacey Holzapfel, Pastoral Extension Officer, Northern Territory Department of Industry, Tourism and Trade.